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The Mechanism and Kinetics of Cyanide-Assisted Carbon Monoxide Desorption from Pt Electrodes: An Infrared Spectroscopic Study Bin Geng, Shaoxiong Liu, Jun Cai, Sangzi Liang, and Yanxia Chen* Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, UniVersity of Science and Technology of China, Hefei, 230026, China ReceiVed: March 21, 2009; ReVised Manuscript ReceiVed: April 27, 2009
Using a thin layer flow cell, we examined the dynamic process of COad desorption at a Pt film electrode in contact with cyanide (CN-) containing solution by time-resolved electrochemical in situ infrared spectroscopy with attenuated total reflection configuration under well-defined mass transport. The time-resolved IR spectral features reveal that starting with a COad saturated layer (i) there is an inverse linear relationship between the band intensities of COL and CN- stretching; (ii) the CN-/COad displacement transient exhibits a “volcano” shape with three distinct kinetic regions including a slow ignition period, fast increase, and subsequent decay in the rate; (iii) the CN-/COad displacement rate decreases with electrode potential. The results suggest that COad desorption is directly related to the nucleation of CN-ad islands on the surface. The mechanism of cyanide-assisted COad desorption is discussed. CO adsorption/desorption at Pt surfaces has been extensively investigated in ultrahigh vacuum environment, at solid/gas and electrode/electrolyte interfaces, encouraged by the desire for a better understanding of the nature of surface bonding as well as by the important role played by adsorbed CO in various industrial relevant processes (e.g., anode reactions in fuel cells, removal of car-exhaust gases, methanation, and Fischer-Tropsch processes).1–4 CO molecules are found to be strongly adsorbing at Pt surfaces and poison the active surface sites. Acceleration of COad desorption is suggested to be one of the two key strategies (in addition to the enhancement of COad oxidation) to alleviate the CO poisoning problems.1–4 To this end, a molecular level understanding of the mechanism and kinetics of CO desorption at a Pt surface is necessary. Previous experiments reveal that COad does not desorb from a Pt electrode surface at room temperature;5 however, as confirmed by 12CO/13COad isotope exchange experiments, the desorption takes place when CO molecules exist in the gas (or solution) phase.6–9 Furthermore, an increase in COad desorption rate with CO partial pressure has been well confirmed.6–9 Conventionally, the desorption of CO from a Pt electrode is believed to be controlled by thermal excitation, and the increase in COad coverage and consequent decrease in CO adsorption energy due to the repulsion between COad-COad molecules is suggested to be the cause for the pressure effect on COad desorption rate. Such a view, however, has been challenged by a new experimental observation that the 12CO/13COad exchange rate only increases ca. 3.5 times with an increase in temperature from 25 to 50 °C under otherwise identical conditions,6,7 while an increase of 6.5 (or 42) times is expected supposing an activation energy of 60 (or 120) kJ/mol for COad desorption and assuming first-order Langmuir desorption kinetics. Tamaru tried to improve the thermal excitation model by including the effect of partial pressure as a perturbation,8 but the use of this model is still * To whom correspondence should be addressed. E-mail: yachen@ ustc.edu.cn.
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rather difficult due to the lack of knowledge about the relationship between activation energy and partial pressure. Due to limited experimental techniques applicable for such a purpose, the real mechanism for COad desorption, as has already been pointed out by Ertl,1 remains to be clarified. In order to better understand this issue, we have monitored the room temperature COad desorption from a Pt electrode/0.01 M KCN + 0.5 M Na2SO4 interface at 0 and -0.4 V (vs RHE) using time-resolved electrochemical infrared spectroscopy with a thin layer flow cell, which allows in situ infrared measurements under well-defined mass transport conditions as well as fast electrolyte switch using a three-way valve connecting the cell and the electrolyte supplying bottles. The results are presented in this communication. We find that starting with a COad saturated layer (i) there is a linear relationship between the loss and gain in band intensities of COL and CN- stretching, (ii) the CN-/COad displacement transient exhibits a “volcano” shape with three distinct kinetic regions, and (iii) the CN-/COad displacement rate decreases with potential. The implications of these results on the mechanism of COad desorption are discussed. Millipore Milli-Q water (18.2 MΩ cm), sodium sulfate, and KCN (AR grade, Shanghai Reagent Corp., China) were used to prepare the solutions. CO saturated solution was prepared by continuous purging of the electrolyte with CO (Nanjing Special Gas Corp., 99.95%) for 30 min. The spectro-electrochemical flow cell used in the present study has been described by Chen et al.9 A 50 nm thick Pt film chemically deposited on the reflecting face of a hemicylindrical Si prism (roughness factor ∼5)9 served as a working electrode. Pt foil and a Hg/Hg2SO4, K2SO4, saturated electrode (MSE) were used as counter and reference electrodes. All potentials in this study are quoted against the RHE. Real-time CN-/CO displacement was monitored by infrared spectroscopy using a Varian FTS 7000e spectrometer with a mercury cadmium telluride detector cooled by liquid nitrogen in the following manner: first, the electrode was held at 0 V (or -0.4 V) in 0.5 M Na2SO4, and a background spectrum (reflectance of R0) was recorded. Then, 2009 American Chemical Society
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Figure 2. Changes of the band intensities of COL (squares) and CN(triangles) at a Pt electrode/0.5 M Na2SO4 interface (a, b) and the inverse of the derivate of the corresponding band intensities as a function of time (c, d) upon switching to 0.01 M KCN + 0.5 M Na2SO4 solution at 0 V (a, c) and -0.4 V (b, d). The insets show an enlargement of the part marked by an ellipse in the band intensity plot.
Figure 1. Temporal evolution of IR spectra of a COad saturated Pt electrode/0.5 M Na2SO4 interface upon switching to 0.01 M KCN + 0.5 M Na2SO4 at (a) 0 V and (b) -0.4 V.
CO saturated solution was flowed through the cell continuously for ca. 10 min to ensure that the surface was fully saturated with COad. Then, the inlet tubes and the cell were carefully cleaned by flushing with 0.5 M Na2SO4 while holding the electrode potential constant. After that, the electrolyte was switched to 0.5 M Na2SO4 + 0.01 M KCN solution. The IR signal was recorded as a function of time with 1 s per spectrum at a resolution of 4 cm-1. All spectra are presented in absorbance, A ) -log(R/R0), where R is the reflectance of the sample spectrum. Figure 1a shows representative sets of IR spectra taken during the solution switch from 0.5 M Na2SO4 to 0.01 M KCN + 0.5 M Na2SO4 at 0 V; the corresponding changes in band frequencies and intensities as a function of time are plotted in Figure 2a. From Figures 1a and 2a, it is seen that the COad saturated Pt surface has two infrared bands, i.e., COL (linearly adsorbed) and COM (bridgeand hollow-site-adsorbed) at 2066 and 1880 cm-1.9 The band intensities are constant before the solution switch, which confirms that CO oxidation or desorption does not occur under the present conditions. Within the first 5 s after flushing the COad saturated Pt surface with 0.01 M KCN + 0.5 M Na2SO4 solution, there is a small decrease (∼2%) in the band intensities which is followed with a sharp decrease with ca. 76% intensity drop and 10 cm-1 red-shift in peak frequency of the COL band from ca. 5 to 45 s. At ca. 9 s, a small band at ca. 2080 cm-1 from the stretching vibration of CN- appears and its peak frequency and band intensity increase with time. (Note previous infrared studies confirmed that at a Pt surface no bridge-bonded or multiply bonded CN- is detected.)10 The opposite change in COad and CN- spectral features reveals the corresponding decrease and increase in the surface coverage of COad and CN-ad species, which demonstrates that, at room temperature, some COad molecules at the Pt surface desorbed upon the flushing of CN- containing solutions at this potential. This is in contrast to the case without CN-, where COad desorption is not observed. From 20 s on, the rate of such spectral changes decreases with time and reaches steady state values at ca. 600 s after solution
Figure 3. Plots of the band intensities of CN- stretching (ACN-) as a function of the band intensities of COL (ACO) observed during solution switch at 0 and -0.4 V (data from Figures 1 and 2).
switch (Figure 2a). The latter suggests that complete removal of COad molecules by elongating the time for CN- exposure is impossible. It is well-known that dipole-dipole coupling interaction among COad species in the adlayer plays an important role in coveragedependent CO spectral features.2 Although CO and CN- are isoelectronic species and the C-O and C-N stretching frequencies only differ ca. 30-50 cm-1, the effect on spectral features from the dipole coupling interaction is found to be negligible, as demonstrated by a recent study on the coadsorption of CO and CN- at a Pt(111) electrode, where barely any difference in CNband intensity between pure CN- saturated adlayer (θ ∼0.5 ML) and CN- + CO saturated adlayer (θt ∼0.5 ML CN- + 0.37 ML CO) is observed.10 On the other hand, several systematic studies reveal that, within the coverage regime from 0.14 to 0.6 ML, there is an approximately linear relationship between the COad surface coverage and COL band intensities,11–13 which allows us to deduce the coverage relationship between COad and CN-ad from their band intensities. The decrease of CO band intensities as a function of a corresponding increase in the CN- band intensities recorded at the same time during the course of CN--CO displacement is plotted in Figure 3, where a rough linear relationship is seen. The nearly one order smaller band intensities of CN- stretching than the corresponding values for COL is rationalized by the difference in the transition dipole moment (µD) for both species. The linear
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relationship suggests that the adsorption of one CN- anion is accompanied with a certain number of COad desorption. In other words, COad desorption is intimately correlated with the adsorption of CN-. The linear θCO-COL band intensity relationship and the negligible interference of spectral behavior by the coadsorbed CNor COad allow us to deduce the kinetics of CN--COad displacement from such time-dependent spectral features. By differentiating the temporal evolution of the spectral features observed during the CN--COad displacement, we obtained a well-defined “volcano” shape of the derivative of CO band intensities as a function of time, as shown in Figure 2c. The volcano-shape displacement transient reveals that there are three distinct periods for the CN--COad displacement kinetics: an initial ignition period of ca. 5 s, followed by a fast increase until the maximum displacement rate is reached (lasts ca. 15 s), and then it decreases again until zero rate is reached (takes more than 500 s). The maximum CN--CO displacement rate is found at ca. 20 s after the solution switch, when the COL band intensity (θCO) drops to ca. 50% of its initial value. The spectral features observed at -0.4 V (Figures 1b and 2b) reveal that the rate for COad removal and CN- uptake at -0.4 V is much slower than that at 0 V, as manifested by the longer ignition period (ca. 20 s) and the slower spectral change with time comparing that at 0 V. Again, we also obtain a “volcano” shape of the derivative of the CO band intensity-time curve (Figure 2d) and a rough linear relationship with the loss of COL band intensity and the increase in CN- band intensities (Figure 3). However, the maximum CN--COad displacement rate appears at much higher COad band intensities (∼80% of the COL intensity at saturation). Of central interest here is what is the mechanism associated with the volcano-type CO desorption kinetics? Though at room temperature thermally excited COad desorption is not observed in CO free solution, rapid surface diffusion of COad molecules occurs. In the initial ignition period, most of the CN- anions which bombard the COad saturated Pt surface are deflected back to solution, since no free space is available for CN- to adsorb. At some special moment, e.g., when a proper open space at the surface is provided by the adlayer reconstruction probably induced by the COad surface diffusion, the incoming CN- has a chance to be adsorbed. It is noticed that at the potentials (0 to -0.4 V) examined in the present study, net charge transfer from CN-ad to Pt and from Pt to COad is expected; thus, the dipole moment of Pt-CN-ad (Pt-COad) is positive (negative).14,15 Once a CN- anion is adsorbed at the Pt surface, a slight lateral shift of its neighboring COad molecules from its original position takes place due to changes in the interparticle interaction.16 This may lead to a decrease of the adsorption energy of COad molecules, and consequently an increase in the COad desorption rate by thermal excitation. In addition, the deviation of COad from its optimum adsorption position provides more open space for the incoming CN- anions to adsorb. As CN- is adsorbing, it may displace adsorbed COad molecules, since the chemical energy released from its adsorption is enough to overcome the COad desorption barrier. Possibly by a combination of these two pathways, the CN--COad displacement rate increases. Like a typical chain reaction, the COad desorption accelerates as such processes proceed. When the surface coverage of CN- is above a certain value, the increased repulsion among CN-ad species leads to a continuous decrease in the rate for the further CN-CO displacement. When changing the electrode potential to more negative values, the sticking coefficient of CN- to the Pt surface decreases due to the electrostatic repulsion; thus, the ignition period takes longer and the subsequent CN--CO displacement rate is slower. Fur-
Letters thermore, due to the increased electronic density at the Pt surface, less negative charge will transfer from CN- anions to the Pt surface than at higher potentials.17 This leads to stronger repulsion among the adsorbed CN- species; as a result, the maximum displacement rate appears at lower CN-ad (higher COad) coverage. Finally, we would like to mention that the CN--COad displacement kinetics differs greatly from that of the 12CO/13COad isotope exchange; in the latter case, a linear decay in the 12CO/13COad isotope exchange rate with the fractional surface coverage of 13COad is observed.18 Furthermore, we do not observe appreciable COad desorption in solutions containing species other than CN- and CO, such as sulfate, halides, and so on. All of these facts imply that thermal excited desorption only contributes partly to the COad desorption processes under the present conditions; instead, “adsorption driven” desorption, i.e, desorption of COad excited by the chemical energy released by CN- or CO to be adsorbed, may play an important role. Further investigations are underway to get further details of such processes. In summary, room temperature COad desorption assisted by cyanide adsorption has been studied by electrochemical infrared spectroscopy. The time-resolved IR spectral data reveals that COad desorption kinetics exhibits a volcano shape with three distinct kinetic regions. The CN--COad displacement transients are intimately correlated with the nucleation of CN- at the surface. CN- adsorption driven COad desorption is suggested to operate for the COad desorption in addition to the thermally excited desorption. Since adsorption/desorption processes are necessary steps for heterogeneous catalytic reactions, we believe that systematic study of the dynamics of such processes will greatly improve our fundamental understanding of such adsorption/ desorption processes and the kinetics of heterogeneous catalytic reactions. We hope the present study will boost wide interest and further investigations on such topics. Acknowledgment. Invaluable discussion with Prof Yi Luo is greatly acknowledged. This work was supported by grants of the National Natural Science Foundation of China (NSFC) (no. 20773116), 100 Talents’ Program of The Chinese Academy of Sciences. References and Notes (1) Ertl, G. Dynamics of reactions at surfaces. AdV. Catal. 2000, 45, 1. (2) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271. (3) Markovic, N. M.; Ross, P. N. Surf. Sci. Rep. 2002, 45, 121. (4) Enger, B., C.; Lodeng, R.; Holmen, A. Appl. Catal., A 2008, 346, 1. (5) Bligaard, T.; Norskov, J. K. Electrochim. Acta 2007, 52, 5512. (6) Davies, J; Nielsen, R., M.; Thomsen, L., B.; Chorkendorff, I.; Logado´ttir, A.; Lodziana, Z.; Nørskov, J., K.; Li, W., X.; Hammer, B.; Longwitz, S. R.; Schnadt, J.; Vestergaard, E., K.; Vang, R., T.; Besenbacher, F. Fuel Cells 2004, 4, 309. (7) Takagi, N.; Yoshinobu, J.; Kawai, M. Phys. ReV. Lett. 1994, 73, 292. (8) Davies, J. C.; Tsotridis, G. J. Phys. Chem. C 2008, 112, 3392. (9) Chen, Y. X.; Heinen, M.; Jusys, Z.; Behm, R. B. Angew. Chem., Int. Ed. 2006, 45, 981. (10) Cuesta, A. J. Am. Chem. Soc. 2006, 128, 13332. (11) Chang, S. C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (12) Chen, Y. X.; Ye, S.; Heinen, M.; Jusys, Z.; Osawa, M.; Behm, R. J. J. Phys. Chem. B 2006, 110, 9534. (13) Heinen, M.; Chen, Y. X.; Jusys, Z.; Behm, R. J. Electrochim. Acta 2007, 52, 5634. (14) Wasileski, S. A.; Koper, M. T. M.; Weaver, M. J. J. Am. Chem. Soc. 2002, 124, 2796. (15) Beltramo, G. L.; Shubina, T. E.; Mitchell, S. J.; Koper, M. T. M. J. Electroanal. Chem. 2004, 563, 111. (16) Barth, J. V. Surf. Sci. Rep. 2000, 40, 75. (17) Ren, B.; Wu, D. Y.; Mao, B. W.; Tian, Z. Q. J. Phys. Chem. B 2003, 107, 2752. (18) Heinen, M.; Chen, Y. X.; Jusys, Z.; Behm, R. J. Chemphyschem 2007, 8, 2484.
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